This invention relates generally to power circuits for providing power to a load with the power coming initially from a first voltage and subsequently from a second voltage when the second voltage reaches a requisite magnitude. The present invention relates more particularly, but not by way of limitation, to apparatus for applying a voltage to an input of a control circuit of a means for converting a direct current input voltage to a regulated direct current output voltage, which applied voltage is initially taken from the input voltage and subsequently from the output voltage.
Devices for converting a direct current input voltage to a lower magnitude direct current output voltage are well known. Such devices include a converter control circuit and one or more switching circuits operated in response to the control circuit to achieve the voltage level conversion. These devices, referred to as DC/DC converters, generally require operating power to be provided at some low stable voltage for their internal control circuits. Such stable low voltage is generally provided through either a series pass regulator circuit or a shunt regulator circuit across which the input voltage is applied.
When one of these types of DC/DC converters has an applied input voltage with a magnitude considerably greater than the stable low voltage which is to be used to power the internal control circuit, significant energy is dissipated in either the series pass or shunt regulation circuit used to achieve this considerably lower voltage. Such dissipation can severely reduce the efficiency of the DC/DC converter. This dissipation can also create high internal operating temperatures or require special heat sinking to prevent such high temperatures from occurring. Such dissipation is a direct function of the difference between the applied input voltage and the lower voltage at which the control circuit power is provided by either the shunt or series pass regulation circuit.
An example of where such relatively high energy dissipation occurs is in instrumentation used in diesel locomotives. Some of the modern instrumentation in such locomotives includes integrated circuits requiring a nominal voltage level of +12 V.sub.DC, for example; however, the voltage supply maintained on a diesel locomotive is at a nominal +72 V.sub.DC. Therefore, such instrumentation must include a DC/DC converter to step the +72 V.sub.DC supply down to the appropriate operating level for the instrumentation. Such a DC/DC converter has an internal control circuit which must be powered at a voltage level comparable to the output level to be generated by the converter (e.g., +12 V.sub.DC). To bring the +72 V.sub.DC supply to this operating level, a conversion circuit of the aforementioned series pass or shunt regulator type has been used; however, this type of circuit when used continuously to power the internal control circuit dissipates substantial energy as a result of the large differential between the +72-volt input and the stable low voltage to be provided by the series pass or shunt regulator circuit for powering the internal control circuit.
Because of this significant energy dissipation resulting from the continuous use of such a regulator circuit, there is the need for a new apparatus which can provide a stable low voltage to power the internal control circuit of a DC/DC converter (or some other suitable load) with less energy dissipation than results from the present technique of continuously utilizing a series pass or shunt regulator circuit to power the load.